Generally, in order to transmit information as a wireless radio wave signal, conversion of a baseband signal (low frequency signal including components near a DC) into a radio high-frequency signal, so-called modulation processing is indispensable. There are roughly three modes of modulation. These three modes include an amplitude modulation (an AM for an analog modulation, and an ASK for a digital modulation) system in which an amplitude of a high-frequency signal (carrier) is changed by a baseband signal, a frequency modulation (an FM for an analog modulation, and an FSK for a digital modulation) system in which a frequency of a high-frequency signal is changed in proportion to a baseband signal, and a phase modulation (a PM in an analog modulation, and a PSK in a digital modulation) system in which a phase of a high-frequency signal is changed in proportion to a baseband signal.
Among these, the amplitude modulation is also called a linear modulation, since information on the amplitude and frequency which the baseband signal has is reproduced by the high-frequency signal as it is. On the other hand, since a spectrum of the modulated high-frequency signal is completely different from that of the baseband signal, the frequency modulation and phase modulation are also called nonlinear modulations. Heretofore, when wireless transmitting a stereo sound signal, the frequency modulation inherently immune to noise on a transmission line has been used (for example, Japanese Patent Laid-Open No. H9-321720, and Japanese Patent Laid-Open No. H7-162383 (patent documents 1 and 2)).